Sophisticated industrial processes require the cooperative execution of numerous interdependent tasks by many different pieces of equipment. The complexity of ensuring proper task sequencing and management requires not only procedural logic, but also constant monitoring of equipment states to organize and distribute operations and detect malfunctions.
Today, many industries use programmable logic controllers to properly operate and monitor elaborate industrial equipment and processes. Industrial programmable logic controllers—PLCs, controllers—operate in accordance with a stored control program that causes the controller to examine the state of the controlled machinery by evaluating signals from one or more sensing devices (e.g., temperature or pressure sensors), and to operate the machinery (e.g., by energizing or de-energizing operative components) based on a procedural framework, the sensor signals and, if necessary, more complex processing.
Ordinarily, process operation is monitored, at least intermittently, by supervisory personnel by means of one or more central management stations. Each station samples the status of controllers (and their associated sensors) selected by the operator and presents the data in some meaningful format. The management station may or may not be located on the same site as the monitored equipment; frequently, one cental station has access to multiple sites (whether or not these perform related processes). Accordingly, communication linkage can be vital even in traditional industrial environment where process equipment is physically proximate, since at least some supervisory personnel may not be.
To facilitate the necessary communication, the controller and related computers (such as monitoring stations) are arranged as a computer network that uses some consistent protocol to communicate with one another. The communication protocol provides the mechanism by decomposing and routing messages to a destination computer identified by an address. The protocol may place a “header” of routing information on each component of a message that specifies source and destination addresses, and identifies the component to facilitate later reconstruction of the entire message by the destination computer. This approach to data transfer permits the network to rapidly and efficiently handle large communication volumes without reducing transfer speed in order to accommodate long individual message.
In typical computer networks having one or more programable logic controllers, a monitoring computer, which may be remotely located from any or all of the controllers to which it has access, periodically queries the controllers to obtain data descriptive of the controlled process or machine, or the controller itself. This data is then available for analysis by the monitoring computer.
The programable logic controllers used today have a very limited amount of memory and thus do not allow a user to customize how the requested information from the controller is displayed. Instead, the monitoring computer receives the requested information and configures it into the display format desired by the user. Likewise, the size and number of programs for execution by a programable logic controller is restricted to the limited memory space within the controller.